The crystal structure of zeolites manifests itself in the form of porous materials, and hence zeolites are used as molecular sieves that let through only molecules of a specific molecular size. By virtue of the characteristic of enabling selective adsorption and desorption of specific molecules, afforded by zeolites, the latter are used for instance in adsorption-type coolers and air conditioners capable of regulating humidity. Coolers and air conditioners in which zeolites are used are advantageous, for instance, in terms of enabling operation using significantly less energy than conventional coolers and air conditioners, and in terms of enabling operation using waste heat. Applications of zeolites are thus being developed in various fields.
Zeolites have to be produced in large quantities in order to be used in such a wide range of applications. In reality, however, the only methods resorted to in practice involve batch production through charging, into an autoclave, of a zeolite starting material (zeolite precursor gel) (hereafter, materials corresponding to a precursor of a zeolite that is subjected to hydrothermal synthesis may be expressed in brackets as zeolite precursor gel, for easier comprehension; the purpose of this notation, however, is to facilitate understanding by making it easier to distinguish the individual starting materials (aluminum source and so forth), and does not alter the meaning of the basic application); production costs are thus high due to material costs and productivity problems. This is one reason why zeolites are not ubiquitously used, despite the excellent energy efficiency that they afford.
Various methods for producing zeolites continuously have been developed conventionally, for the purpose of enhancing zeolite productivity. For instance, PTL 1 discloses a technique that involves introducing a microwave synthesis method, as a measure for shortening a protracted crystallization process, which is one fundamental problem of hydrothermal synthesis, so that it becomes possible to shorten synthesis time through activation of water and ions in a synthesis solution. This technique allows synthesizing a zeolite within 5 minutes, through heating using microwaves in the range of 60 to 1200 W. This can be combined with an effect of making it possible to shorten the production time through addition of 0 to 20% of a molecular sieve. In a case where microwaves are utilized, however, a material such as a fluororesin has to be unavoidably used as part of the reactor, since microwaves do not pass through metals like stainless steel. Such resin materials were problematic, in terms of safety, in hydrothermal synthesis carried out under high pressure.
As a method for synthesizing a zeolite continuously in a tubular reactor that is heated with a heat medium, NPL 1 discloses an example of continuous synthesis of silicalite through oil bath-heating using a stainless-steel capillary reactor. This method enables continuous synthesis of silicalite, with a retention time of 5.8 minutes, using a stainless-steel capillary reactor heated to 150° C. Synthesis in accordance with this method, however, is problematic in that the particle size of the zeolite that is obtained is small, lying in the range from 10 to 100 nm, and the zeolite is difficult to recover by ordinary solid-liquid separation such as filtration. The obtained zeolite, moreover, comprises a significant amount of amorphous component, which translates into poor crystallinity and low quality.
Further, PTL 2 discloses a method for synthesizing a zeolite continuously in a tubular reactor that is heated with a heat medium. For instance, PTL 2 (paragraphs [0046] and [0047]) discloses the feature of generating turbulence within the tubular reactor, to promote the reaction thereby. The zeolite generation reaction can be efficiently carried out as a result.
Further, NPL 2 illustrates examples of synthesis of A-zeolite, Y-zeolite and silicalite-1 zeolite using a tubular reactor having a diameter of 6 mm, heated in an oil bath. In the case of continuous synthesis of a zeolite using a tubular reactor having a diameter of 3 cm or smaller, as illustrated in NPL 2, the use of continuous synthesis was however limited to zeolites, typified by A-zeolite and Y-zeolite, that crystallize from a low-viscosity starting material mixture (zeolite precursor gel) in which no template is used, or to zeolites, typified by silicalite-1 zeolite, that crystallize from a low-viscosity transparent solution. In terms of ease of crystallization as well, the use of continuous synthesis was limited to, for instance, A-zeolite or Y-zeolite, which are synthesized easily to the extent of not requiring any template, or to silicalite-1 zeolite (MFI-type, framework density 17.91/A3) having a high framework density and which can crystallize easily. In cases of zeolite crystallization from a starting material mixture (zeolite precursor gel) comprising aluminum and phosphorus, and cases of zeolite crystallization from an aluminosilicate mixture (zeolite precursor gel) comprising a template, however, there have been no examples of continuous synthesis in a tubular reactor having a diameter of 3 cm or smaller, in accordance with a heating method in which an ordinary heat medium is resorted to, due to the high viscosity of the starting material mixture (zeolite precursor gel).
It is deemed that also zeolites of low framework density, i.e. having large spaces in the crystal, are difficult to synthesize continuously in accordance with the above method, from the viewpoint of crystallization difficulty. As reasons underlying this assertion, it is firstly found that stirring of a starting material (zeolite precursor gel) during hydrothermal synthesis is important, as suggested by PTL 2; accordingly, a zeolite that uses a high-viscosity starting material (zeolite precursor gel) cannot be made. Secondly, it is found that a reaction tube becomes clogged readily when synthesizing the zeolite in a high-viscosity starting material (zeolite precursor gel).